JPH0341935B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable gain image intensifier having an image intensifier tube.

The following explanation essentially refers to RII (Radiological
It relates to a radiographic image intensifier tube, also known as an image intensifier tube. However, it is clear that the invention also applies to photointensifiers and scintillation scanning image intensifiers (gamma radiation).

The RII tube preferably has a variable gain target whose gain factor (corresponding to the number of photons emitted for each electron applied to the target) is approximately 10
It can be doubled. Thus, if desired, the RII tube can function for X-ray imaging or fluoroscopy.

In X-ray photography, the RII video output signal is
It makes it possible to display the information contained in the X-ray beam reaching the tube on a television screen, and the television image is recorded on film or a photograph. In order to obtain a good signal-to-noise ratio (SN ratio), intense X-rays must be irradiated over a short period of time.
It is necessary to have a low gain target to prevent saturation.

In X-ray fluoroscopy, the television screen is directly observed and is irradiated with weak X-rays for a relatively long observation period. In this case, it is necessary to have a high gain target to obtain a good image.

In this manner, in X-ray imaging, since strong X-rays are irradiated in a short period of time, the target does not need to have a high gain, but has a low gain to prevent saturation. on the other hand,
In X-ray fluoroscopy, the target is required to have a high gain because the observation is performed over a long period of time and weaker X-rays are irradiated to minimize radiation exposure to the observer. The difference in required target gain between radiography and fluoroscopy is approximately 100 times.
That is, since fluoroscopy requires 100 times as many photons to be applied to the target for each electron, the target gain for fluoroscopy must be 100 times the target gain for radiography.

French Patent Application No. 7705031, published as No. 2341939, discloses an RII target with a variable gain video output. In reality it is a silicon target, one side of which is covered with a luminescent coating which is itself covered with a metal barrier layer.

The electron beam from the RII cathode reaches a metal barrier layer, which slows down the electron beam and only allows higher energy electrons to pass through. In luminescent coatings, these electrons contribute to the formation of photons, which generate charge carriers in the target silicon. These charge carriers discharge oppositely connected diodes located on the other side of the target. Finally, the charge distribution on the other side of the target is scanned by the electron beam of the imaging tube to provide a video signal.

Target gain changes are obtained by varying the accelerating voltage of the RII beam and by using the nonlinear relationship that exists for metal barrier layers between electron penetration into the barrier layer and electron beam accelerating voltage. .

However, this prior art variable gain target has the following drawbacks. That is, RII
The resolution of the tube is reduced by the use of two layers covering the silicon target, namely a metal barrier layer and a luminescent coating, the presence of the metal barrier layer causing noise and as described in page 2, line 6 of the French patent application. ~Line 21 and 5
As explained in lines 27 to 31 of the page, defects occur in the obtained image.

The present invention provides an image intensifier with a variable gain target that eliminates the above-mentioned problems.

The present invention relates to an image intensifier having an image intensifier tube, the image intensifier including means for allowing the electron beam from its photocathode to be subjected to two different accelerating voltages.

The target included in the image intensifier of the present invention includes two types of luminescent materials that have different luminous efficiencies and emit light when an electron beam is applied. An electron beam subjected to a lower accelerating voltage excites only luminescent substances with a lower luminous efficiency, and an electron beam subjected to a higher accelerating voltage excites luminescent substances with a higher luminous efficiency.

Therefore, the gain of this target can be changed significantly by applying the high and low acceleration voltages to the different luminous efficiencies of the two luminescent substances constituting the target. Metal barrier layers are not used which would cause noise and image defects.

According to a preferred embodiment of the invention, the two luminescent materials of the target each emit light of different wavelengths, and the target has a matched optical filter that is configured to emit light from the other luminescent material. The light emitted from the luminescent material, which has a higher luminous efficiency, is transmitted more than the light emitted by the luminescent material.

Therefore, a target with a gain of about 100 times can be obtained.

Furthermore, in another preferred embodiment of the invention, two
Two luminescent materials are supported by a fiber optic board, and the target is made of silicon and covered with a luminescent coating and a metal barrier layer. This provides better resolution than in the prior art.

The invention will be explained in more detail below by means of non-restrictive examples and the accompanying drawings, in which: FIG.

Although the same reference numerals refer to the same components in the drawings, the dimensions and dimensions of the various parts have not been taken into account.

FIG. 1 is a schematic block diagram of an RII with a video output designated generally by 1. FIG. From left to right in the figure, first there is an RII (radiation image intensifier) tube, and then there is an image pickup tube, both of which are housed in the same vacuum container 2.

After passing through the observed object 3, the X-ray beam enters the RII tube through the window 4.

The RII tube consists of an input screen composed of a scintillator 5 and a photocathode 6 that convert X-rays into photons and then photoelectrons, and grids g ₁ , g ₂ and g ₃ that focus electrons and apply accelerating voltages. The electron optical system includes a conical anode A, and a target 7 which receives the electron beam impingement on its surface _f1 .

The other surface _f2 of the target 7 is scanned line by line by an electron beam generated by a cathode K heated by a filament 8 of the image pickup tube. This electron beam is
It is focused and accelerated by grids _g4 to _g7 . The beam is concentrated and deflected by a coil (not shown). An output video signal S is collected on target 7.

FIG. 2 is a diagram showing an embodiment of a target in the present invention. This target is constituted by an optical fiber board having a length of, for example, 2 mm to 5 mm.

FIG. 3 shows in detail the surface f ₁ of this target, that is, the surface located on the RII tube side. It can be seen in FIG. 3 that each optical fiber of the board has a blind hole obtained by removing the core 12 of the optical fiber to a depth of, for example, 5 μm without touching the coating 13. For example, this can be obtained by selective chemical etching of the two glasses forming the core 12 and the coating 13. In this way, a blind hole with a depth of 5 μm and a diameter of 5 μm, for example, separated by walls of 2 μm, for example, is obtained.

Inside each hole, first a layer L 2 of particulate luminescent material with a higher luminous efficiency r ₂ is deposited, then a barrier layer 14 and a layer L ₂ with a lower luminous efficiency r ₁
Deposit another layer of granular luminescent material L ₁ with . Note that the term "luminescent substance" as used herein refers to a substance that emits cold light, such as a fluorescent substance or a phosphorescent substance.

Before filling the holes in the manner described above, the side walls of each hole are covered with a thin metal coating 15. Typically, this sidewall consists of aluminum deposited by vacuum evaporation. When performing this vacuum deposition, aluminum can be deposited correctly on the sides of the hole by appropriately selecting the direction in which the aluminum vapor enters.
When the hole is filled, the upper side of layer L ₁ is further covered with a thin metal coating 15 .

The RII tube includes a manual or automatic switching device, which has means for causing the electron beam from the photocathode to be subjected to two mutually different accelerating voltages V ₁ and V ₂ , for example 10 kV and 30 kV. are doing.

Luminescent materials with lower luminous efficiency
As only L ₁ is excited by the electron beam subjected to the lower accelerating voltage V ₁ , therefore the luminescent substance L ₂ with higher luminous efficiency is mainly excited by the electron beam subjected to the higher accelerating voltage V ₂ . The luminescent substances L ₁ and L ₂ and the third
The thickness of the barrier layer 14 shown is selected.

Other means by which the above results can be obtained are described below. These means differ from the aforementioned means in that, for example, in the absence of a barrier layer, and in that the luminescent substances are not particulate, but instead are in the form of transparent thin layers obtained by vacuum deposition of their constituent materials. is different.

FIG. 4 shows the variation of brightness L as a function of accelerating voltage for materials L ₁ and L ₂ . Since the current density of the incident beam is constant, the brightness increases with the accelerating voltage from the threshold V ₀₁ for L ₁ and from the threshold V ₀₂ for L ₂ , and so on. This increase in brightness is faster for _L2 than for _L1 .

When the electron beam from the photocathode of the RII tube reaches the surface f ₁ of the target 7, it passes through the metal coating 15 and the luminescent material L ₁
enters the first layer.

When this beam is exposed to a lower accelerating voltage V ₁ , a portion of the beam's electrons, say 50%,
does not pass through L ₁ and the other 50% does not pass through barrier layer 14.

Excitation of layer _L1 generates a relatively small amount of light due to the low luminous efficiency of this layer. As previously mentioned, the outer surface of layer L ₁ and the side walls of each blind hole are coated with a thin metal coating 15 so that the light emitted from layer L ₁ of each optical fiber is directed along this optical fiber to target 7. propagates toward the other surface f ₂ . There is no light scattering and the same resolution as that of the fiber optic board is maintained.

If the beam is subjected to a higher accelerating voltage V ₂ ,
Some of the electrons in the beam, say 15%, do not pass through the layer _L1 , another part of the electrons, say 35%, do not pass through the barrier layer, and these remaining electrons pass through the layer L with higher luminous efficiency. Excite ₂ .

It is clear that the amount of light emitted for accelerating voltage V ₂ is greater than the amount of light emitted for accelerating voltage V ₁ . V ₁ = 10kV and V ₂ = 30kV
When the luminous efficiency r ₂ /r ₁ is approximately 5, a gain of approximately 20 times can be obtained. By using luminescent materials L ₁ and L ₂ emitting light of different wavelengths λ ₁ and λ ₂ , respectively, e.g. red and green, the luminous efficiency is even higher than the light emitted from other luminescent materials L ₁ By using a matched optical filter that transmits more of the light emitted from _the luminescent material L ₂ with , the gain due to the layer of luminescent material L ₂ is approximately 100
It is possible to obtain a variable gain that doubles.

Granular luminescent material that emits red light L ₁
may be composed of europium-doped yttrium oxide sulfide or europium-doped yttrium oxide, for example with a grain size of 1 μm or less. The granular luminescent substance L ₂ that emits green light has a particle size of 2 μm, for example.
It can be composed of doped cadmium zinc sulfide. Layer L ₁ is a single layer with a thickness of 1 μm or less, and layer L ₂ has a thickness of 4 μm, for example.

FIG. 5 shows that the transmission coefficient T of a matched optical filter as described above varies as a function of wavelength λ.

The layers are arranged such that the gain due to layer L ₂ is 100 times the gain due to layer L ₁ , or higher if necessary.
The transmission coefficient T ₁ of the filter for light from L ₁ is λ ₁
The transmission coefficient T ₂ of the filter for light from layer L ₂ is matched to a value according to λ ₂ . It is well known that the overall gain including the filter is the product of the luminous efficiency of the luminescent material and the transmission coefficient at the wavelength of the emitted light.

The light emitted from the luminescent materials L ₁ and L ₂ travels along the optical fiber to the opposite side of the target 7.
Propagates up to f ₂ . The structure of this target 7 is
It is easily understood from the figure.

The face _f2 of the target 7 is covered with a thin transparent conductive coating 9 formed by vacuum deposition. This coating 9 may consist of tin oxide SnO ₂ , indium oxide In ₂ O ₃ , cadmium oxide CdO ₃ , manganese oxide MnO, or mixtures of these oxides. To obtain a gain of 100 times, the coating 9 is covered by a matched optical filter 10.

The optical filter is formed by depositing a material in a very thin coating, less than 1 micron, and the transmittance can be varied by varying the thickness of the coating in a known manner. For example, it is possible to deposit lutetium diphthalocyanide.

In a conventional image pickup tube, the target 11 is deposited on the filter 10 and may consist of a continuous photoconductive layer or a diode connected with opposite polarity or the like. This photoconductive layer is made of an amorphous compound of antimony sulfide, amorphous amorphous, selenium telluride,
These include sulfur and arsenic, or lead oxide layers.

This target is read line by line by the electron beam of the image pickup tube.

It is clear that if it is only necessary to obtain a gain of about 20 times, a luminescent material emitting light of the same wavelength can be used and a matched optical filter need not be used.

According to another embodiment of the invention, the surface f ₂ of the optical fiber board, like the surface f ₁ , has three layers 9, 10, 11.
It has a blind hole filled with.

6, 7, and 8 are target plane f ₁
Other examples are shown in which the targets are all comprised of fiber optic boards.

In FIG. 6, each optical fiber has a blind hole as in FIG. In each hole first a layer of granular luminescent material L ₂ with a high luminous efficiency r ₂ is deposited, and then a deposited layer L ₁ of luminescent material with a low luminous efficiency r ₁ is deposited.

The deposited layer L ₁ may be selected such that there is no need to insert a barrier layer between layers L ₁ and L ₂ . Also, the lower accelerating voltage V ₁ only excites layer L ₁ , and the higher accelerating voltage V ₂ excites layer L ₂ .

The deposited layer L ₁ has about 100 when passing from V ₁ to V ₂
A sufficiently low luminous efficiency can be provided such that no additional matching optical filter is required to obtain double the gain.

In order to preserve the resolution of the fiber optic board as in FIG. 3, the side walls of the blind hole and the outer surface of layer _L1 are covered with a thin metal coating 15.

FIG. 7 shows an example of a target surface f ₁ in which the surface of the board is covered with two deposited layers L ₁ and L ₂ of luminescent materials with different luminous efficiencies. Barrier layer 1 similarly obtained by vacuum deposition
4 can be placed between layer L ₁ and layer L ₂ if necessary. A thin metal coating 15 covers the outer surface of the layer L ₁ of low luminous efficiency.

By using thin coating layers L ₁ , L ₂ and 15, each obtained by vacuum deposition of the constituent materials, a target with excellent resolution is obtained without the need for drilling optical fibers.

In the example of target surface f ₁ shown in FIG. 8, the optical fiber core 12 protrudes from the surface of the board. This is obtained, for example, by selective chemical etching of the two glasses constituting the core and the cladding, respectively, similar to obtaining the blind holes in the examples of FIGS. 3 and 6. However, in this case, removing the coating from the optical fiber was a problem.

As in FIG. 7, on the surface of each core two deposited layers L ₁ and L ₂ of luminescent materials with different luminous efficiencies are deposited. A thin metal coating 15 covers layer _L1 , and a vapor deposited barrier layer can be used if necessary.

The layer L ₁ may be composed of europium-doped yttrium oxysulfide oxide;
L ₂ may be composed of yttrium oxysulfide doped with terbium. These two layers are deposited using an electron gun according to conventional techniques.

According to another embodiment of the invention, not shown in the figures, the target is constructed, for example, by a silicon semiconductor substrate rather than an optical fiber board. The silicon surface is covered with two layers L ₁ and L ₂ , which are preferably vapor-deposited layers of non-granular luminescent material to improve the resolution.

According to a further embodiment of the invention, two stacked luminescent material layers selectively separated by a barrier layer are no longer used. Instead, two types of particulate luminescent materials with different luminous efficiencies are used. The grains of these two materials are mixed and one grain of material is covered with a barrier layer.

9 and 10 illustrate two examples of RII tubes with video outputs containing targets according to the present invention. Unlike the example shown in FIG. 1, the RII tube 20 and the imaging tube 21 are placed in two separate vacuum containers.

In the example of FIG. 9, the RII pipe has a target 7 as shown in FIG.
The planes f ₁ and f ₂ are connected to several layers L ₁ , L ₂ , 15 and 9,
It is composed of optical fiber boards covered with 10 and 11.

For example, color 2 such as pyroceramic sealing.
2, the imaging tube container is fixed to the RII tube container. As a result, there is no need to expose the imaging tube to the high temperatures required to make RII tubes. Furthermore, it is possible to test the operation of the RII tube before installing the imager tube.

In FIG. 10, an RII tube 20 and an image pickup tube 21 are joined by two separate optical fiber boards 22 and 23. In FIG.

According to a preferred embodiment of the invention, the RII tube side board 22 has layers L ₁ , L ₂ on its left hand side f ₁ , as shown for example in FIGS. 3 and 6-8.
and 15, and the image pickup tube side board 23 has layers 9, 10 and 11 on its right-hand side _f2 .

[Brief explanation of drawings]

Figure 1 has a video output according to the prior art
A schematic configuration diagram of RII; FIG. 2 is an explanatory diagram of an embodiment of the target in the present invention; FIGS. 3 and 6-
FIG. 8 shows several embodiments according to the present invention of the target surface receiving the electron beam from the RII tube.
4 is an illustration of the variation in brightness as a function of accelerating voltage for the luminescent substances L ₁ and L ₂ ; FIG. 5 is an illustration of the variation in the transmission coefficient of an optical filter as a function of wavelength. 9 and 10 are illustrations of two embodiments of RII tubes having video outputs containing targets according to the present invention. 2...Vacuum container, 3...Object, 4...Window, 5...
... scintillator, 6 ... photocathode, 7 ... target, 12 ... optical fiber core, 14 ... barrier layer, 15 ... metal coating.

Claims (1)

[Scope of Claims] 1. A photocathode for generating a first electron beam in response to applied X-rays, means for focusing the first electron beam on an electron beam receiving target, and a photocathode for generating a first electron beam in response to applied X-rays; a variable gain image intensifier comprising a radiographic image intensifier tube comprising means for accelerating at a first high voltage or a second lower voltage;
The target includes an optical fiber board having a first surface for receiving the electron beam and a second surface opposite to the first surface for receiving a second electron beam for scanning; a first surface of the optical fiber, the first electron beam being disposed proximate the first surface of the optical fiber and receiving the first electron beam and generating light that travels from the first surface proximate through the optical fiber board to the second surface; 1 and a second luminescent material, the first and second luminescent materials have a higher luminous efficiency and a lower luminous efficiency, respectively, and the first electron beam When receiving a second accelerating voltage, which is the lower of the two accelerating voltages, a luminescent material having a lower luminous efficiency is excited, and the first electron beam is receiving the higher of the two accelerating voltages. An image intensifier of variable gain, characterized in that the luminescent material having high luminous efficiency is arranged to be excited when receiving a first accelerating voltage on the side. 2. the two luminescent materials of the target emit light having different wavelengths, and the target has a matched optical filter, the optical filter having a wavelength higher than that of the light emitted from the other luminescent material. An image intensifier according to claim 1, characterized in that it transmits more light emitted from a luminescent material having luminous efficiency. 3. Claims characterized in that said two luminescent substances are contained in a blind hole obtained by drilling a hole in the core of an optical fiber, the side walls of said hole being covered with a thin metal coating. Image intensifier according to paragraph 1. 4. The blind hole contains two layers of particulate luminescent material separated by a barrier layer, the layer of luminescent material with higher luminous efficiency being disposed at the bottom of the hole and the layer with lower luminescent efficiency 4. Image intensifier according to claim 3, characterized in that the layer of efficient luminescent material is covered with a thin metal coating. 5. said blind hole comprises a layer of granular luminescent material with high luminous efficiency coated with a deposited layer of luminescent material with low luminous efficiency, said deposited layer being covered with a thin metal layer; An image intensifier according to claim 3, characterized in that: 6. The surface of the optical fiber board is coated with two deposited layers of luminescent material, the first layer has a high luminous efficiency and the second layer has a low luminous efficiency, the second layer has a Claim 1 characterized in that it is coated with a thin metal layer coating.
Image intensifier as described in section. 7. The core of the optical fiber protrudes from the surface of the optical fiber board, and the core of the optical fiber is 2
the first layer has a high luminous efficiency and the second layer has a low luminous efficiency, and said second layer is coated with a thin metal coating. An image intensifier according to claim 1, characterized in that: 8. Image intensifier according to any one of claims 5 to 7, characterized in that a barrier layer is inserted between two layers of luminescent material. 9. Image intensifier according to claim 1 or 2, characterized in that the semiconductor substrate contains two luminescent substances. 10. An image intensifier as claimed in claim 1, including a photosensitive image tube target on the side opposite the side coated with luminescent material. 11. An image intensifier according to claim 9, characterized in that it includes a photosensitive image tube target on the side opposite to the side coated with the luminescent material. 12. An image intensifier according to claim 10 or 11, characterized in that an optical filter is arranged in front of the photosensitive image tube target. 13 The second surface of the optical fiber board is
Claim 1, further comprising a photosensitive material disposed near the second surface of the optical fiber of the optical fiber board to receive the scanning second electron beam. The image intensifier described in . 14. An image intensifier according to claim 10 or 11, characterized in that it includes an image pickup tube that emits an electron beam that scans the photosensitive imaging target. 15. The image intensifier according to claim 14, wherein the image intensifier and the image pickup tube are arranged in the same vacuum container. 16. The image intensifier of claim 14, wherein the image intensifier and the image pickup tube are each located in two separate vacuum vessels. 17. The image intensifier of claim 1, wherein the target by the fiber optic board is coupled to the image pickup tube by another fiber optic board.